1. Field of the Invention
The present invention relates to an image forming apparatus and a toner supply method.
2. Description of the Related Art
In the related art, a single-component developing apparatus is known. In the single-component developing apparatus, developing is carried out in such a manner that a developing roller, which is a toner carrying member, carries toner that is developer of a non-magnetic or magnetic single component, and the toner on the developing roller is supplied to a latent image on a photosensitive member, in a developing area in which the photosensitive member that is a latent image carrying member and the developing roller face one another.
In the single-component developing apparatus, the developing apparatus is replaced when the toner in the developing apparatus has run out. Therefore, the developing roller for which a replacement time has not been reached yet and thus usage of which can be continued is also replaced. Thus, a resource may be wasted. In a case where the developing apparatus is configured such that a time at which the toner in the developing apparatus runs out and the replacement time of the developing roller and so forth are the same as one another, it may be necessary to ensure a space for holding a great amount of toner in the developing apparatus, and thus, the developing apparatus may be increased in size.
Japanese Patent No. 4026977 (patent document 1) describes an image forming apparatus in which a toner container that holds toner is provided separate from a developing apparatus, and a supplying part supplies the toner of the toner container to the developing apparatus. Thereby, in a case where the toner in the toner container has run out, only the toner container is to be replaced, and thus, a developing roller for which usage can be continued is not to be replaced. Further, because an amount of new toner to be held by the toner container may be determined without regard to a replacement time of the developing roller, it is possible to reduce a capacity of the toner container to be small, and thus, it is possible to prevent the image forming apparatus from being increased in size.
Further, according to the image forming apparatus of the patent document 1, the toner of the toner container is supplied to the developing apparatus when the toner amount in the developing apparatus becomes less than a lower limit value. Therefore, after the toner is thus supplied to the developing apparatus, the toner newly supplied from the toner container and old toner having remained in the developing apparatus for a long period of time without being used for developing are mixed together.
Japanese Laid-Open Patent Application No. 2009-75244 (patent document 2) describes the following image forming apparatus for preventing fog caused as a result of new toner being supplied to a developing apparatus in which old deteriorated toner remains. The term “fog” means a phenomenon that an image area that is to be a blank has a density increased as a result of toner adhering thereto through a developing process. That is, when a toner amount in the developing apparatus becomes less than a lower limit value, such control is carried out that, before toner is supplied to the developing apparatus, the toner remaining in the developing apparatus is ejected toward a photosensitive member. Thereby, the old toner remaining in the developing apparatus is ejected to the photosensitive member, and, in a condition in which the developing apparatus has thus become approximately empty of toner, new toner is supplied from a toner container. Therefore, almost all of the toner in the developing apparatus becomes the new toner after the new toner is thus supplied, and thus, it is possible to prevent fog after the new toner is supplied.
However, according to the above-mentioned patent document 2, the old toner remaining in the developing apparatus is ejected to the photosensitive member, and thus, is discarded. As a result, the toner may be wasted.
A reason why fog occurs when deteriorated old toner and new toner are mixed will now be described concretely.
Old toner remaining in a developing apparatus has suffered stress for a long period of time due to such as stirring. As a result, an external additive that is added externally to surfaces of toner particles for controlling flowability and an electrification property may have been removed or may have been embedded in the particles. Thereby, the toner may not be easily electrified frictionally to, for example, negative polarity that is normal electrification polarity of the toner. On the other hand, new toner supplied to the developing apparatus is not deteriorated and thus, is easily electrified frictionally to the negative polarity. Therefore, when the new toner that is easily electrified to the negative polarity and the old toner that is not easily electrified to the negative polarity are rubbed together, charge separation occurs, and electrons in the old toner move to the new toner. As a result, an electrification amount of the new toner having the negative polarity may increase, an electrification amount of the old toner having the negative polarity may decrease, or the old toner may be electrified to have positive polarity. As a result, the toner electrification distribution in the developing apparatus becomes broad, and also, such a distribution may occur in which two peaks, i.e., an area in which the electrification amount to the negative polarity is large and an area in which the electrification amount is close to zero, exist. Thus, after the new toner is supplied, the deteriorated toner may become weak electrified toner, or reverse electrified toner. Therefore, in an image forming process after the new toner is supplied, the above-mentioned deteriorated old toner may adhere to an area (other than a latent image area) on a photosensitive member that is an image carrying member for which area no toner is desired to be placed. As a result, fog increases in comparison to a case before the new toner is supplied.
Such fog can be reduced by reducing a ratio of deteriorated old toner existing after new toner is supplied. Therefore, by increasing a new toner amount to be supplied, it is possible to reduce the ratio of deteriorated old toner in the developing apparatus without carrying out the above-mentioned process of ejecting the toner remaining in the developing apparatus, and thus, it is possible to reduce fog occurring after new toner is supplied without uselessly consuming the toner.
However, when the new toner amount to be supplied is increased, the toner amount in the developing apparatus increases, which may result in an increase in torque of a stirring member, toner packing or such. Therefore, it is necessary to reduce, as much as possible, the toner amount in the developing apparatus after new toner is supplied. If the remaining old toner amount in the developing apparatus can be precisely determined when new toner is supplied, it is possible to supply a minimum necessary amount of new toner to the developing apparatus such that fog occurring after the new toner is supplied can be made to be an allowable level, and it is possible to prevent the toner amount in the developing apparatus after the new toner is supplied from becoming larger than the necessary amount. The term “toner packing” means a phenomenon that toner solidifies, or a phenomenon that fluidity of toner remarkably degrades. Toner packing may occur as a result of toner being pressed at high pressure, and may result in an increase of a torque of a machine that processes the toner or a structure that passes the toner therethrough is clogged by the toner.
A remaining amount detecting part that detects a toner remaining amount in the developing apparatus in the related art is such that a piezoelectric sensor is provided in the developing apparatus and a remaining amount in the developing apparatus is detected by using an output of the piezoelectric sensor. Another example is such that a detecting window is provided to the developing apparatus, a transmission optical sensor is used to detect whether toner exists in the detecting window to detect a height of the toner in the developing apparatus, and a remaining amount is detected from the detected height of the toner in the developing apparatus. However, in the remaining amount detecting method of using the piezoelectric sensor, precise remaining amount detection may not be carried out when a deviation occurs in a toner amount between a place at which the piezoelectric sensor is provided and a place at which the piezoelectric sensor is not provided. Further, in the remaining amount detecting method of using the transmission optical sensor, precise remaining amount detection may not be carried out when a deviation occurs in a toner amount between a place at which the transmission optical sensor is provided and a place at which the transmission optical sensor is not provided. Thus, it may be difficult to precisely determine a toner remaining amount in the developing apparatus in the related art, and thus, it may not be possible to supply, based on a detection result of the remaining amount detecting part, a minimum necessary amount of new toner to the developing apparatus such that fog occurring after the new toner is supplied can be made to be an allowable level. Therefore, new toner may be supplied in an amount larger than the necessary amount, and thus, it may not be possible to sufficiently prevent an increase in torque of a rotating member such as a stirring member, toner packing, or such in the developing apparatus.